1. Field of the Invention
This invention relates to combined stop and intercept valves for steam turbines.
2. Description of the Prior Art
In large steam turbine generator sets, it is necessary to control the amount of steam admitted to the steam turbine to control the speed thereof; or, in an emergency situation, such as the loss of the electrical load on the generator, to shut down the turbine completely. To achieve this control of steam turbine speed, it has been the practice to provide steam valves for controlling steam flow from a high-pressure steam turbine to a low-pressure steam turbine, these valves being individually actuated in response to power plant conditions. Under normal operating conditions, both a stop valve (upstream) and an intercept valve (downstream) are opened providing for a maximum amount of steam flow between the high-pressure and low-pressure turbines. During a turbine overspeed condition, when the turbine speed reaches 103% of rated speed, the intercept valve closes substantially all the way to shut off the supply of steam to the low-pressure turbine. The stop valve backs up the intercept valve. Should the turbine speed reach 108% of rated speed, the stop valve closes completely in response to a general turbine trip, shutting off the supply of steam to the low-pressure turbine. Should the overspeed condition be relieved before the general turbine trip and the resulting closing of the stop valve, the intercept valve may be opened restoring the supply of steam to the low-pressure turbine without shutting down the turbine.
Prior art combined stop and intercept valves are of a cup and plunger type. Such a cup and plunger type combined stop and intercept valve, which includes a generally annular seat engageable by a cup-shaped intercept valve element and a plunger-shaped stop valve element, has certain disadvantages. One such disadvantage is that it is necessary to turn the steam flow through the valve through a 90.degree. angle, removing substantial quantities of available energy from the steam flow. In addition, the cup-shaped and plunger-shaped members cannot be formed in an aerodynamic shape, contributing further to the losses associated with this prior art valve. The cup and plunger type combined stop and intercept valve is also costly to manufacture.
To overcome the deficiencies of prior art cup and plunger type combined stop and intercept valves, the present invention provides a combined stop and intercept valve employing butterfly discs as both stop and intercept valve elements and arranged in line with the steam flow between the high and low-pressure turbines. Butterfly valves have been used in the past to control the flow of steam to a steam turbine. However, in prior art serial arrangements of two or more butterfly discs, the downstream disc exhibited a fluttering in the wake of the fluid flowing around the upstream disc, removing substantial amounts of available energy from the flow.
Therefore, it is an object of this invention to provide an improved stop and intercept valve disposed between and in fluid communication with a high-pressure and a low-pressure steam turbine wherein a downstream valve element will not be affected by a wake of the steam flowing around an upstream valve element thereby preventing the removal of available energy from the steam flow.
It is another object of this invention to provide an improved combined stop and intercept valve disposed between and in communication with a high-pressure and low-pressure steam turbine wherein changing the direction of the steam flow through the valve is eliminated thereby avoiding losses of available energy from the steam flow.
It is another object of this invention to provide an improved combined stop and intercept valve disposed between and in fluid communication with a high-pressure and a low-pressure steam turbine wherein the valve elements are of aerodynamic shape minimizing the removal of available energy from the flow of steam through the valve.
It is another object of this invention to provide an improved combined stop and intercept valve disposed between and in fluid communication with a high-pressure and a low-pressure steam turbine which is less expensive to produce than prior art combined stop and intercept valves.